The development of surgical techniques have made great progress over the years. For instance, for patients requiring brain surgery, non-invasive surgery is now available which is afflicted with very little trauma to the patient.
Stereotactic radiosurgery is such a minimally invasive treatment modality that allows delivery of a large single dose of radiation to a specific intracranial target while sparing surrounding tissue. Unlike conventional fractionated radiotherapy, stereotactic radiosurgery does not rely on, or exploit, the higher radiosensitivity of neoplastic lesions relative to normal brain (therapeutic ratio). Its selective destruction depends primarily on sharply focused high-dose radiation and a steep dose gradient away from the defined target. The biological effect is irreparable cellular damage and delayed vascular occlusion within the high-dose target volume. Because a therapeutic ratio is not required, traditionally radioresistant lesions can be treated. Because destructive doses are used, however, any normal structure included in the target volume is subject to damage.
One such non-invasive radiotherapy technique is so called LINAC (Linear Accelerator) radio therapy. In a LINAC radiotherapy system, a collimated x-ray beam is focused on a stereotactically identified intracranial target. In such an accelerator, electrons are accelerated to near light speed and are collided with a heavy metal, e.g. tungsten. The collision mainly produces heat but a small percentage of the energy is converted into highly energetic photons, which, because they are electrically produced, are called “x-rays”. The gantry of the LINAC rotates around the patient, producing an arc of radiation focused on the target. The couch in which the patient rests is then rotated in the horizontal plane, and another arc is performed. In this manner, multiple non-coplanar arcs of radiation intersect at the target volume and produce a high target dose, resulting in a minimal radiation affecting the surrounding brain.
Another system for non-invasive surgery is sold under the name of Leksell Gamma Knife®, which provides such surgery by means of gamma radiation. The radiation is emitted from a large number of fixed radioactive sources and are focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, towards a defined target or treatment volume. Each of the sources provides a dose of gamma radiation which is insufficient to damage intervening tissue. However, tissue destruction occurs where the radiation beams from all radiation sources intersect or converge, causing the radiation to reach tissue-destructive levels. The point of convergence is hereinafter referred to as the “focus point”. Such a gamma radiation device is, for example, referred to and described in U.S. Pat. No. 4,780,898.
In the system, the head of a patient is immobilized in a stereotactic instrument which defines the location of the treatment volume in the head. Further, the patient is secured in a patient positioning system which moves the entire patient so as to position the treatment volume in coincidence with the focus point of the radiation unit of the system.
Consequently, in radiotherapy systems, such as a LINAC system or a Leksell Gamma Knife® system, it is of a high importance that the positioning system which moves the patient so as to position the treatment volume in coincidence with the focus point of the radiation unit of the system is accurate and reliable. That is, the positioning system must be capable of position the treatment volume in coincidence with the focus point at a very high precision. This high precision must also be maintained over time.
Hence, in order to obtain as favorable clinical effect as possible during the therapy is it of an utmost importance that the radiation reaches and hits the target, i.e. the treatment volume, with a high precision and thereby spares the healthy tissue being adjacent to and/or surrounding the treatment volume. To achieve this, the patient must be immobilized during a therapy session and, moreover, the position of the head of the patient must be the same in a therapy session as in a reference position, i.e. the position during the session when the pictures to create the therapy plan were captured by means of, for example, Computerized Tomography Imaging (CT-imaging). In fractionated radiotherapy where the patient is docked in and out of the radiation therapy system at each therapy session, it must thus be secured that the patient is positioned in exact the same way as in the session when the pictures were captured to create the therapy plan.
One prior art method for enabling measurements of the head of a patient and for fixating the head of the patient during neurological diagnosis, therapy or surgery, in particular during radiation therapy relatively a frame adapted to be fixated to a radiation therapy unit is a stereotactic frame provided with pin support members in form of posts having fixation pins for invasive fixation to the skull of a patient. In use during for example MRI (Magnetic Resonance Imaging) diagnostics, the stereotactic frame is arranged around the head of a patient, and the fixation pins of the posts connected to the frame are screwed into or to abutment against the bone of the skull, thus ensuring a rigid fixation of the reference system. The frame is then rigidly held in position in relation to a MRI table. This kind of frame is obviously not suitable for so called fractionated therapy.
Thus, there is a need within the art of improved means that enables accurate and fast measurements of a position of a head of a patient relative a radio therapy unit to secure that the patient is positioned in exact alignment to a reference position or at a known position in relation to the reference position.